You are getting good advice. Try it first. You need to shock your pool as suggested (use bleach or liquid chlorine--easiest to use and least side effects) to kill and metabolize everything.

Then you need to get all the dead stuff out. The DE improves sand filter performance, but ONLY add enough to raise pressure 1 pound. Start with a 1/3 of a cup poured slowly into the skimmer, wait 20 min and check pressure. Repeat until it's up a pound. If you accidentally go too high (say, 5 pounds) and it's clogging up, simply backwash to get the DE out and start again.